There is an ongoing need for component miniaturization in radio wave communication devices. For example, smaller and more efficient components are needed for light-weight, hand-portable cellular telephones, wireless local area networks for linking computer systems within office buildings in a readily reconfigurable fashion, wristwatch- and credit-card-sized paging apparatus and other devices for promoting rapid, efficient and flexible voice and data communication.
Filters are needed for a variety of such communications applications wherein small size, light weight and high performance are simultaneously required. Increasing numbers of products seek to employ fixed spectral resources, often to achieve tasks not previously envisioned. Examples include cellular telephones, inter- and intra-facility computer-computer and/or computer-ancillary equipment linkages as well as a host of other, increasingly complex inter-personal and/or -equipment information sharing requirements. The desire to render increasingly complicated communications nodes portable and even hand-held and/or -portable and/or pocket-sized places extreme demands on filtering technology in the context of increasingly crowded radio frequency resources.
Acoustic wave filters provide filters meeting stringent performance requirements which are (i) extremely robust, (ii) readily mass produced, (iii) adjustment-free over the life of the unit and which (iv) sharply increase the performance to size ratio achievable in the frequency range extending from a few tens of megahertz to about several gigahertz. However, need for low passband insertion loss simultaneously coupled with demand for high shape factor and high stopband attenuation pose filter design and performance requirements not easily met by a single acoustic wave filter alone.
One approach to satisfying these needs and demands is to cascade two or more acoustic wave filters. This approach realizes increased stopband signal rejection but requires additional matching components (e.g., inductors and/or capacitors) and also multiplies the volume and weight of the acoustic wave filters by the number of such filters cascaded. Matching components additionally incur major size and weight penalties because each transducer generally requires at least two matching components, each of which is at least as large as an acoustic wave filter die.
Another approach is to provide two or more such filters on a single substrate, wherein the filters are designed to have purely real impedances matched one to another without requiring intervening matching components. One realization includes a series-parallel arrangement of resonant elements having staggered center frequencies and arranged in a ladder structure, i.e., a structure comprising cascaded sections each including a series resonant element followed by a shunt resonant element. Typically, within each section, the antiresonant frequency of the shunt element is chosen to be the resonant frequency of the accompanying series element, providing pure real input and output impedances. Disadvantages of this approach when implemented employing SAW resonators include a fixed bandwidth for the electromechanical coupling coefficient (k.sup.2) associated with the chosen substrate material. Prior art ladder filter structures employing piezoelectric series and/or shunt elements have generally been confined to extremely narrow bandwidths, on the order of 0.01% to 0.1%, in part because they have been implemented with piezoelectric materials having very low electromechanical coupling coefficients. Generally, conventional approaches are such that when three of the filter material, impedance, selectivity and bandwidth characteristics are specified, the fourth is also determined.
What is needed is a ladder filter configuration/design methodology providing flexible bandwidth, suitable out-of-band rejection and low in-band insertion loss, not requiring external matching components, drift-free performance and realizable in compact, monolithic form.
Acoustic wave filters employ generally periodic arrays of electrodes configured to provide discrete elements such as transducers (for converting electrical to mechanical energy and vice versa), reflectors (for reversing the direction of propagation of an acoustic wave) and gratings (e.g., for separating transducers and/or resonant cavities and/or providing electrical isolation therebetween). These elements are grouped in a generally in-line configuration (e.g., reflector, transducer, grating, transducer, reflector) and are separated by inter-element gaps, with the entire array providing an electrical filtering function associated with the electrical port(s) of the transducer(s). The relative widths of the inter-element gaps affect the electrical and acoustic performance of the composite filter.
The desired electrical performance often dictates gap sizes representing deviations from the periodicities of the respective elements, providing surface boundary condition discontinuities and reflecting a portion of the incident acoustic wave and scattering another portion into bulk acoustic waves. Particularly on high electromechanical coupling coefficient substrates, surface boundary condition discontinuities lead to bandwidth perturbations, increased in-band (i.e., passband) insertion loss and decreased out-of-band (i.e., stop-band) signal rejection.
What are needed is an apparatus and a method for making acoustic wave filters having electrical performance characteristics that are less degraded by bulk scattering losses even when implemented on high coupling coefficient substrates.